The present invention relates to materials for aligning liquid crystals, and liquid crystal optical elements.
Current liquid crystal display elements include a product that utilize a twisted nematic mode, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 90xc2x0 between a pair of upper and lower electrode substrates, a product utilizing a supertwisted nematic mode, utilizing a birefringent effect, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 180xc2x0 to 300xc2x0, an in-plane-switching mode wherein both electrodes controlling the liquid crystal alignment are present on one substrate and the direction of the liquid crystal orientation in the plane of the substrate changes upon application of an electric field, and a product utilizing a ferroelectric liquid crystal substance or an antiferroelectric liquid crystal substance. Common to each of these products is a liquid crystal layer disposed between a pair of substrates coated with a polymeric alignment layer. The polymeric alignment layer controls the direction of alignment of the liquid crystal medium in the absence of an electric field. Usually the direction of alignment of the liquid crystal medium is established in a mechanical buffing process wherein the polymer layer is buffed with a cloth or other fiberous material. The liquid crystal medium contacting the buffed surface typically aligns parallel to the mechanical buffing direction. Alternatively, an alignment layer comprising anisotropically absorbing molecules can be exposed to polarized light to align a liquid crystal medium as disclosed in U.S. Pat. Nos. 5,032,009 and 4,974,941 xe2x80x9cProcess of Aligning and Realigning Liquid Crystal Mediaxe2x80x9d which are hereby incorporated by reference.
The process for aligning liquid crystal media with polarized light can be a noncontact method of alignment that has the potential to reduce dust and static charge buildup on alignment layers. Other advantages of the optical alignment process include high resolution control of alignment direction and high quality of alignment.
Requirements of optical alignment layers for liquid crystal displays include low energy threshold for alignment, transparency to visible light (no color), good dielectric properties and voltage holding ratios, long-term thermal and optical stability and in many applications a controlled uniform pre-tilt angle. Most liquid crystal devices, including displays, have a finite pre-tilt angle, controlled, for instance, by the mechanical buffing of selected polymeric alignment layers. The liquid crystal molecules in contact with such a layer align parallel to the buffing direction, but are not exactly parallel to the substrate. The liquid crystal molecules are slightly tilted from the substrate, for instance by about 2-15 degrees. For optimum performance in most display applications a finite and uniform pre-tilt angle of the liquid crystal is desirable.
Continuing effort has been directed to the development of processes and compositions for optical alignment of liquid crystals and liquid crystal displays. Through diligent effort and intensive experiments we have found that the alignment quality and electrical properties, specifically the voltage holding ratio, of polyimide alignment layers derived from polyamic acids containing the novel crosslinking diamines disclosed herein are significantly improved over those lacking such crosslinking capability.
The present invention provides a process for preparing an optical alignment layer for aligning liquid crystals comprising: preparing a layer of polymer selected from the group consisting of polyamic acid, polyamide ester and polyimide, comprising anisotropically absorbing molecules, on a substrate; exposing said layer to polarized light; the polarized light having a wavelength within the absorption band of said anisotropically absorbing molecules; wherein the resulting exposed anisotropically absorbing molecules induce alignment of a liquid crystal medium at an angle with respect to the major axis of the polarization of the incident light and along the surface of the optical alignment layer; wherein the polymer is further characterized in having side chains selected from the group of C3-C22 linear and branched hydrocarbon and partially fluorinated hydrocarbon chains, optionally substituted by one to two heteroatoms selected from the group consisting of N, S, O and having one to four carbon-carbon double bonds.
Further embodiments of the invention are polymers including polyimides, polyamic acids and esters thereof, characterized in that they comprise identical or different repeat units selected from one or more of the formula 
wherein B is hydrogen or a monovalent organic group derived from an alcohol after formal removal of the hydroxyl group, M is a tetravalent organic radical of a tetracarboxylic acid anhydride after formal removal of the two xe2x80x94COxe2x80x94Oxe2x80x94COxe2x80x94 groups, the four valencies of which are distributed between four different carbon atoms of the radical; A is a trivalent unsubstituted or optionally fluoro-, chloro-, cyano-, alkyl-, alkoxy-, fluoroalkyl- or fluoroalkoxy- substituted aromatic or alicyclic radical; Y is selected from the group of C3-C22 linear and branched hydrocarbon and partially fluorinated hydrocarbon chains, optionally substituted by one to two heteroatoms selected from the group of N, S, O and having one to four carbon-carbon double bonds.